A conventional printed wiring board is a board with a metallic foil affixed to a resin substrate made from a composite material such as epoxide woven glass, epoxide cellular paper, phenolic cellulose paper or polyimide woven glass, and for example, affixing copper foil thereto, a circuit board is formed by etching the copper foil in a predetermined pattern. Electronic components are arranged in predetermined positions on the circuit board, and electrode terminals of the electronic components are connected by soldering to electrode terminals of the circuit board, thereby electrical connecting and fixing of the components in order to manufacture an electronic circuit device. In such a method, the resin substrate is required to endure the temperature (about 250° C.) for soldering. Also, it is necessary to make equipment investment for a reflow device, cleaning device or exhaust gas treating equipment. Further, since lead (Pb)-tin (Sn) type solder widely used in general involves an environmental problem, solder that contains no lead such as tin (Sn)—silver (Ag)-copper (Cu) is used for soldering. However, the melting temperature of such solder is in many cases higher than that of conventional eutectic solder, and the resin substrate is further required to be higher in heat resistance.
On the other hand, the prevalence of mobile equipment such as portable telephones in particular is very remarkable, and further, for achieving the purposes of reduction in size and weight and higher function, there is a strong demand for high performance and high-density mounting with respect to a circuit board as well. Accordingly, in order to mount as many electronic components as possible in a limited space in a portable telephone or the like, it is becoming necessary for a flexible printed circuit board.
Further, with the recent prevalence of digital camera, compact disk (CD) drive or digital versatile disk (DVD) drive, components such as plastic lenses which are relatively low heat resistance are often mounted on a circuit board. Such components of low heat resistance cannot be subjected to reflow soldering together with ordinary electronic components. Therefore, a conventional method is such that general electronic components including semiconductor elements are connected by reflow soldering, while components of low heat resistance such as plastic lenses are connected by using conductive resin paste. Thus, the components are mounted separately, and there arises a problem that the mass productivity cannot be improved.
On the other hand, the following method is disclosed in Japanese Laid-open Patent S62-2593. That is, conductive resin paste with conductive particles contained in resin is used to form a circuit pattern on the surface of a circuit board where electronic components are mounted. Subsequently, as the conductive resin paste is not dried, electronic components are arranged in such manner that the electrode terminals of the electronic components come in tight contact with the conductive resin paste surface that serves as the electrode terminals on the circuit pattern. After that, the conductive resin paste is cured by heating, thereby forming the finally cured circuit pattern and at the same time making the electrical and mechanical connection with the electronic components. In this method, since conductive resin paste is used, it is not necessary to be heated up to the solder reflow temperature, and a thin circuit module having a smooth surface can be realized.
Also, the following method is disclosed in Japanese Laid-open Patent H3-136290. A plurality of electronic components are arranged in such manner that the electrode terminals of these electronic components are flush with each other, which are integrally formed and secured with resin molding material including the surfaces of electronic components other than electrode terminals. Exposed electrode terminal surfaces and circuit patterns for connecting these electronic components to each other are formed by printing with use of conductive resin paste. In such a method, various electronic components can be formed together with the circuit pattern of the board by using conductive resin paste instead of soldering. Accordingly, it is possible to mount various electronic components at a relatively low temperature the same as in the above method.
However, in the first example, the electronic components are arranged while the conductive resin paste of the circuit pattern on the board is not cured, and after that the conductive resin paste is cured to make the electrical and mechanical connection with the conductive resin paste. However, when general conductive resin paste is used, it is unable to increase the adhesion between the electronic component and the circuit board. Therefore, the occurrence of disconnection between the terminals is prevented by covering the electronic components with ultraviolet setting resin.
Also, in the second example, a plurality of electronic components are connected and formed together with circuit patterns by using conductive resin paste, but it is executed after burying the electronic components in resin molding. When the electronic components are buried in resin molding material, positional deflection is liable to take place between the electronic components. In case of such positional deflection, it will often cause positional deflection between the electrode terminal of the electronic component and the circuit pattern when printed. Particularly, when many electronic components are arranged or the electronic component used has many terminals, the influence of positional deflection is remarkable. That is, in such a method, it is difficult to form circuit patterns at fine pitches and to mount electronic components in high density.
The purpose of the present invention is to provide an electronic circuit device which is inexpensive and excellent in mass-productivity and capable of mounting various electronic components together including electronic components of weak heat resistance, and a manufacturing method for the circuit device.